In the incineration of garbage, the solid residues which are formed include so-called grate ash, namely, the materials which are cast off traveling grates or materials which pass through traveling grates or stationary grates and thus are collected below or downstream of the combustion grate, boiler ash which is generally collected from the flue gas passages, and filter dusts which are recovered from flue gas cleaning and can include the fly ash.
These residues include noncombusted carbon, soluble metals and their compounds and halogenated hydrocarbons like dioxins, furanes and precursors thereof, all of which are detrimental to the solid residues and can contribute to the inability to dispose of them in a safe, sanitary and environmentally sound manner.
The proportion of such undesirable components is less in the case of grate ash than for both of the other two residues and decreases with decreasing particle size diameter of the residues.
The use of grate ash in highway or road construction has been proposed. Fly ash filter dust, however, have been disposed of in landfills because they are less useful as components of structural materials or in highway construction. Before such landfill disposal, however, the products must be treated to reduce the content of noxious or toxic components or leachable substances.
The various methods of treating fly ash, for example, include a thermal treatment with oxygen deficiency to break down the halogenated hydrocarbons (DE-OS 37 03 984) and the so-called 3R process in which the fly ash is treated together with scrubbing water from a flue gas scrubber to decompose metal compounds and enable the scrubbed fly ash to be fed back to the combustion chamber (see DE-OS 33 20 466).
Various melting processes are known. For example DE-OS 37 16 231 describes a process in which the fly ash is melted with a plasma burner to eliminate metals and toxic hydrocarbons.
Boiler ash is usually withdrawn from the grate ash but poses a problem because of its much smaller particle size.
In EP-A 2 241 635, the treatment of grate ash is described in which a fine fraction (particle size less than 0.8 mm) is separated out and the coarse fraction is reused. The reprocessing and disposal of grate ash and optionally boiler ash has created resistance in the field because of the presence of toxic substances which are water soluble. In some cases such disposal has been made illegal and in other cases environmental activists have increasingly resisted the disposal processes.
The process and an apparatus for the treatment of so-called slag (grate ash) and so-called fly ash (boiler ash and filter dust) has been described by H. Ruegg in (Thome-Kozmiensky, K. J. (Hrsg.) Recycling Berlin '79, Band 1, EF-Verlag, Berlin 1979, P. 571-574. In this case, the slag cascades from a grate of a refuse-burning plant directly into a melting chamber in which it is melted by heat produced by oil burners supplied with air. The liquefied slag flows into a water bath. The fly ash can be melted in a combustion chamber with a burner and the molten fly ash can also be fed to a water bath. The flue gas is fed to the combustion chamber.
The combustion chamber, the melting chamber and the water bath of this slag-melting apparatus is open to the fire box of the boiler and, as a consequence, the burner supplied energy is contributed with a low operating efficiency.
It is also a drawback of this system that the residence time of the residue in the melt space cannot be easily controlled. Reliable melting of the residues, complete destruction of the halogenated hydrocarbons and elimination of all of the toxic and noxious substances also cannot be guaranteed by this process.
From WO 90/03856, another process for the treatment of slags and other combustion residues from garbage incineration is known and uses separate melting furnaces. The slag and other combustion residues are heated in a preheating device and fed to the melting furnace. From the melting furnace, the slag is fed to a cooling device.
In a separate melting furnace, the residence time of the residues in the melt of bath can be better controlled and the decomposition of halogenated hydrocarbons and the complete combustion of unburned carbon can be guaranteed or made more reliable. In this system, however, only the metals with higher vapor pressure which evaporate during the melting operation and which are entrained with the waste gas from the furnace can be fully removed from the residues. Residual metal remains and, depending upon its melting point and on the residence time, can appear in liquid or solid form in the melt and poses a detriment to the product withdrawn from the melting furnace.
Reference may also be made to U.S. Pat. Nos. 4,353,713, 4,448,588 and 4,597,771, which deal with the processing of gases, solid municipal wastes and the like.